The phenomenon of magnetostriction pertains to the change exhibited in a material's dimensions or shape when it is exposed to an external magnetic field. Another perspective on such behavior is that these magnetostrictive materials convert magnetic energy into mechanical energy, as exposure to the magnetic field induces deformation of the material, which is known as the Joule Effect, and which may be measured using a strain gauge to determine the relative displacement of particles in the object (i.e., the ratio of elongation with respect to the original dimension). Where a magnetostrictive material is induced by the Joule effect to elongate in a lengthwise direction, through rotation and reorientation of small magnetic domains therein—regions of uniform magnetic polarization—a corresponding decrease in the material dimension is experienced in the transverse direction, resulting in negligible changes to its volume. Increases to the strength of the magnetic field applied to the material further increase the induced strain (μL/L-units of micro-length per unit length), until its saturation value (λ) is reached, at which generally all the magnetic domains of the material are aligned with the applied magnetic field.
The reverse phenomenon also exists, where a mechanically induced change to the magnetostrictive material's dimensions creates a corresponding magnetic field, in what is referred to as the Villari Effect. This bi-directional coupling between magnetic and mechanical states is inherent to the particular material, and does not degrade with the passage of time, although a ferromagnetic material that is induced to undergo magnetostriction will not naturally be restored back to it initial magnetization state, after the magnetic field is removed, which reflects the magnetic memory of that material. Certain compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are termed “permanent magnets.” In general, energy must be supplied to drive back the magnetic domains in the material, by the imposition of a magnetic field in the opposite direction. This reluctance to retraceability is known as the hysteresis loop.
The magnetostrictive effect of certain materials (e.g., nickel) were productively used in early applications such as telephone receivers and torque meters, and the Villari effect is commonly used in contact-less sensors. While copper exhibits the greatest room temperature magnetostriction of any pure element and saturates at 60 microstrains, the 1970s discovery of “giant” magnetostrictive alloys (the alloying of elements whose magnetostrictive behavior results in strains greater than 1000 μL/L when exposed to small magnetic fields), is now successfully utilized as sound and vibration sources, for vibration controls, for motional controls, and for materials processing (see, “Handbook of Giant Magnetostrictive Materials,” by Goran Engdahl). Currently, the greatest room-temperature magnetostriction of an alloy is shown by Terfenol-D—a combination of terbium (Ter), iron (Fe), and the rare earth element dysprosium (D). Terfenol was developed by the Naval Ordnance Laboratory (NOL), and exhibits roughly 2000 microstrains in a magnetic field of 2 kOe (160 kA/m), so it is the most commonly used magnetostrictive material in engineering applications.
However, while the magnetostrictive effect may be harnessed for beneficial uses, there are also unproductive results and drawbacks. A common side effect is the annoying humming sound resulting from the 60 Hz applied magnetic field on an AC electrical transformer, where a maximum change in length occurs twice per cycle, producing a 120 Hz humming sound plus harmonics. A similar humming may be heard around high power electric lines carrying alternating current. In addition, a significant counterproductive result also occurs where magnetic devices, such as small high voltage power supplies, transformers, and inductive components, must be securely mounted or must be encapsulated, and because of this constraint on its dimensional changes, a loss in efficiency of the device results.
In applications where an external pressure is applied—either by restriction of deformation by a rigid encapsulant or contraction due to polymerization of a rigid encapsulant, or by the mere use of the magnetostrictive device in a high pressure application—it is desirable to surround the magnetic device with a suitable compressible media. Closed cell foams have been used in those applications where space and size permit. However, in small assemblies, particularly for microelectronics, use of the prior art foams and methods is not feasible. The present invention discloses a new coating that has been tested on microelectronics to successfully counter such losses in efficiency, and furthermore discloses specific application techniques/requirements.